In dust-collecting electrostatic precipitators, the provision of feedthroughs for high voltages involves various difficulties even if the precipitator is operated under conventional conditions, i.e. under an atmospheric pressure .+-.200 mbars and at temperatures up to 150.degree. C.
For instance, care must be taken to ensure that the insulators remain free of moisture or dustlike deposits so that creep currents on the insulator and resulting flashovers will be avoided.
That problem has often been attacked by scavenging the insulator with a protective gas which prevents an ingress of contaminated gases and also maintains the insulator at a constant temperature. Various proposals of that kind have been published in German Pat. Nos. 351,076, 463,528, 1,093,447 and 2,914,241.
The provision of feedthroughs for high voltages becomes even more difficult when substantial pressure differences exist between the interior of the dust-collecting electrostatic precipitator and its environment. In that case it is not sufficient for the feedthrough to be insulating and gastight but it must also be strong enough to withstand the pressure applied and it must prevent an escape of gas even when the insulator has been damaged. Feedthroughs meeting said requirements have been proposed in German Pat. Nos. 550,699 and 886,327; German Utility Model No. 1,830,056; and German Pat. No. 2,556,546.
Further difficulties arise when the feedthrough is exposed to high temperature. In such cases most of the soft elastic sealing materials can no longe be used and in addition to the differential thermal expansions of the insulating materials it is necessary to take substantial changes of resistivity into account.
For instance, the resistivity of a conventional ceramic insulating material decreases from 10.sup.14 ohms/cm to 10.sup.8 ohms/cm and in case of a special insulating material decreases from 10.sup.18 ohms/cm to 10.sup.11 ohms/cm in response to a temperature rise from 20.degree. C. to 200.degree. C. Correspondingly larger insulators must be used in such cases and this gives rise to additional problems in design and manufacture.
Since much higher operating voltages are permissible when dust is electrostatically collected under higher pressures and at higher temperatures, it is particularly important in these caess to solve the problems of feedthrough insulators.
Finally, the design of feedthroughs for high voltages is also difficult because the rapping blows produced outside the housing must be transmitted to the corona electrodes which are suspended in the housing by the insulators. Because of the primary use of ceramic insulators which are particularly susceptible to impact stresses it is necessary to ensure that the rapping blows will be transmitted by the feedthroughs into the interior of the housing in such manner that the insulators will not be subjected to mechanical impact loads.